1. Field of the Invention
The present invention concerns a method of purifying lithium hexafluorosphate that allows purifying lithium hexafluorophosphate, useful as, for instance, lithium secondary cell electrolyte, organic synthesis medium or the like, to an extremely high purity.
2. Description of the Related Art
Lithium hexafluorophosphate is usually synthesized in an insoluble medium such as hydrofluoric anhydride or organic medium; however it reacts with a trace of water existing in the medium, and generates oxyfluoride and lithium fluoride etc., then they are mixed into the product and contaminates the same. Besides, lithium hexafluorophosphate decomposes and equilibrates partially as shown in the following expression, when it is dissolved in hydrofluoric anhydride or organic medium.LiPF6←→LiF+PF5
This LiF is mixed into the product and becomes an impurity.
As lithium hexafluorophosphate has an extremely high moisture absorption ability, even a trace of humidity in the atmosphere provokes hydrolysis to generate the fluoric acid portion. In addition, lithium fluoride or lithium oxyfluorophosphate (LiPOxFy) is generated to cause an impurity. If lithium hexafluorophosphate containing such impurities is used as lithium cell the electrolyte, fluoric acid erodes into electrode material, and lithium fluoride becomes a residue as it is insoluble in organic medium and requires filtration and removal, while lithium oxyfluorophosphate accelerates the decomposition of the organic medium, and possibly increases inner voltage or lowers cell capacity.
Conventionally, these impurities can be removed, by a method wherein lithium hexafluorophosphate is dissolved in an organic medium and impurities are removed by an ion exchange resin (Japanese Patent Laid-Open No. SHOU 59-87774) or a method for neutralizing in an organic medium (Japanese Patent Laid-Open No. SHOU 59-81870); however, these methods increase the production process and costs.
Moreover, a method for reacting and removing oxyfluorophosphate with fluoric gas (Japanese Patent Publication No. HEI 4-16406, Japanese Patent Publication No. HEI 4-16407) or a method for making hexafluorophosphate which contacts with a gas containing pentafluorophosphate (Japanese Patent Laid-Open No. HEI 6-29850) or the like involves danger in gas treatment and require special knowledge. Moreover, fluoric gas or phosphor pentafluoride to be used are required to be refined to a high purity, increasing cost and considerably affecting the product price.
Therefore, none of the conventional methods are satisfactory.
It is an object of the present invention to provide a method of purifying lithium hexafluorosphate that allows to obtain high purify lithium hexafluorophosphate, by converting hydrogen fluoride, oxyfluoride, lithium fluoride or other impurities, generated in the course of production, handling or conservation of lithium hexafluorophosphate and mixed with the product, into lithium hexafluorophosphate.